* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Soil-forming factor 1: Parent material Parent material refers to material from which the soil has been derived and, in most cases, is of geological origin (see peat on page 30). The nature of the parent material can have a profound influence on the characteristics of the soil. For example, the texture of sandy soils is determined largely by the parent materials, which in turn controls the movement of water through the soil. The mineralogy of the parent material is mirrored in the soil and can determine the weathering process and control the natural vegetation composition. For example, lime-rich soils are generally derived from calcareous rocks (e.g. limestone, chalk) or sediments derived from such deposits. In turn, lime-rich soils can offset the development of acidic conditions but may not support plants that are not tolerant of alkaline soil conditions (e.g. rhododendrons). Two types of parent material are recognised: a) unconsolidated deposits or loose sediments that have been transported by ice, water, wind or gravity and b) weathered materials directly overlying the consolidated hard rock from which they originate. In both cases, the parent material can be weathered through physical destruction of rock (freezing or drying cycles) or chemical reactions (dissolution of elements). Weathered parent material is often referred to as saprolite. While the forces created by the expansion and contraction of minerals, induced by daily temperature variations, cause rocks to shatter and exfoliate (especially in hot deserts), in most cases water is the dominant agent in weathering processes. Water can cause rocks to shatter through repeated freezing and thawing of water trapped in rock cavities. Water also initiates solution and hydrolysis (the destruction of a compound through a reaction with water that produces an acid and a base) that liberate minerals contained within the rock. Water also supports life which, in certain situations, is a major contributor to the weathering process. Plant roots can cause physical weathering as they grow and expand inside cracks in the rocks. Roots and decaying vegetation also produce organic compounds such as solvents, acids and alkalines that enhance the actions of percolating rainwater. The degree of weathering depends on a number of environmental factors such as temperature (determined by climate, exposure and altitude), the rate of water percolation (determined by texture, relief, climate), the presence of oxygen (again texture and climate), the surface area of the parent material (largely determined by the geological structure) and the mineralogy of the parent material (for example, quartz is much more stable than olivine). Examples of soil formations on continuous, hard bedrock in Tunisia (left; GT) and transported sediments in East Africa (right; EM), in this case, of fluvial origin. Where consolidated parent material lies close to the surface, soil depth is generally shallow and horizon development is weak. Unconsolidated sediments can completely mask the characteristics of the underlying bedrock. The map below shows the lithological properties of the surface geology of Africa. Lithology describes the mineral composition and structure of geological material which is based on rock formation (i.e. whether it is igneous, sedimentary, metamorphic) and mineralogy (e.g. carbonate, silicic, mafic). This map is a good proxy for soil parent material as it only reflects surface conditions and not the underlying bedrock. It should be noted that the general nature of this map means that at a local level, the conditions may be quite different to that shown. Other than the terms alluvium (deposited by water), aeolian (deposited by wind), organic (peat deposits) and colluvium (transported by gravity), all of which denote recent deposition, the age of material is not indicated. The preponderance of wind-blown sediments across Africa is striking as are the volcanic areas. Please consult the glossary at the end of the atlas, the adjacent text box and page 28 for an explanation of the geological terms. (USGS/JRC) [8a] Weathering continues in the soil following a sequence from the least to the most stable minerals. For example, • Early weathering: clay fraction of young soils, often characterising dry conditions Gypsum, Calcite, Hornblende, Biotite • Intermediate weathering: temperate soils, often with natural grass or forest vegetation Quartz, Muscovite, Vermiculite, Montmorillonite • Advanced weathering: clay fraction of highly weathered soils of humid/tropical regions Kaolinite, Gibbsite, Hematite, Goethite Minerals undergo changes that cause the formation of secondary minerals and other compounds that are soluble in water (to varying degrees). The distinctive features for Africa from a geological point of view are four-fold: • most of Africa is made up of very old rock, often exposed to the surface. In most cases it is crystalline and derived from igneous sources (i.e. magma) and has often been altered chemically (metamorphised); • the continent is slowly being torn in two along the great East African Rift Valley - volcanic activity is common; • large parts of the African land surface have been geologically stable for millions of years which gives rise to deep weathering profiles; • large parts of Africa are covered by recent sediments, in particular, wind blown sand. A thorough summary of the geology of Africa is given in . Surface lithology Carbonates Non-carbonate sedimentary Metamorphic sedimentary Basic igneous rock Acid igneous rock Metamorphic igneous rocks Extrusive volcanic rocks and ejecta Colluvium Organic material Aeolian sediments Alluvium Water 14 Soil Atlas of Africa | Introduction Rock types Igneous rocks are formed by the solidification of molten magma and are the original source of all other rocks. They include rocks such as granite, dolerite and basalt and are generally divided according to the presence of the mineral quartz. Sedimentary rocks are formed by the deposition of weathered material by wind or water. Shales are deposited on ocean floors. Conglomerates and sandstones are composed of resistant fragments of other rocks while limestones and chalk are created through the precipitation of calcium carbonate from solution. Metamorphic rocks are igneous or sedimentary rocks that have been transformed by intense heat or pressure resulting in changes in mineralogy and structure. Examples include gneiss, marble and slate.